Stupid question - How does a torque wrench work?

Edit - I know what a torque wrench is used for and I know when and how to use it. I'm just interested in how it works internallyThis question sounds idiotic but I feel like car people may know...but how does a torque wrench work?Say I have a digital one and I set it to 100ft/lbs. How does it "know" when I torque down that hard? If I lightly twist it vs if I really give it my all, what is being determined to give it that famous "click" of the wrench? via /r/cars http://ift.tt/2rNtXKg